The hydrogenation reaction of organic compounds has been used in a variety of chemical arts. For example, the cracking reaction of petroleum involves the hydrogenation of heavy oil to gasoline or kerosine. Some hydrogenation reactions have been put to practical use. For example, tar is hydrogenated to a liquid so that it is more suitable for further applications. Furthermore, hydrogenation is used to convert unsaturated hydrocarbon to a corresponding saturated hydrocarbon.
This hydrogenation reaction normally proceeds in a uniform system. For example, an organic compound is reacted in the presence of a contact catalyst in a reaction cell while supplying hydrogen. A noble metal such as palladium is known as an excellent catalyst for the hydrogenation of unsaturated hydrocarbon (S. Siegel, "Comprehensive Organic Synthesis", ed., B. M. Trost and I. Fleming, Pergamon Press, Oxford, 1991, vol. 8). This hydrogenation reaction uses high pressure hydrogen and thus requires the use of a high pressure vessel. Further, this hydrogenation reaction mostly takes place at a relatively high temperature. Moreover, this hydrogenation reaction is disadvantageous in that the hydrogen that is used may explode depending on its purity. This hydrogenation reaction is also disadvantageous in that the catalyst has an insufficient reaction selectivity, causing the production of by-products.
In order to enhance the reaction selectivity and reduce the consumption of energy, an electrolytic reduction method involving a nonuniform reaction has been employed as described in A. M. Couper, D. Pletcher and F. C. Walsh, "Chem. Rev.", 1990, [90], 837, T. Nonaka, M. Takahashi and T. Fuchigami, "Bull. Chem. Soc. Jpn.", 183 [56], 2584, M. A. Casadei and D. Pletcher, "Electrochim. Acta", [33], 117 (1988), T. Yamada, T. Osa and T. Matsue, "Chem. Lette,", 1989 (1987), L. Coche, B. Ehui, and J. C. Moutet, "J. Org. Chem.", [55], 5905 (1990), and J. C. Moutet, Y. Ouennoughi, A. Ourari and S. Hamar-Thibault, "Electrochim. Acta", [40], 1827 (1995).
The use of an electrode catalyst having a large surface area such as a Raney nickel catalyst allows for an electrochemical hydrogenation reaction. Thus, good power efficiency can be expected. Further, operation can be effected easily and safely. However, in order for the organic reaction to proceed electrolytically, the organic compound itself must be electrically conductive. Otherwise, additives must be added to render the electrolyte containing an organic compound electrically conductive. Since most organic compounds are nonconductive, the addition of additives to the organic compounds complicates the reaction system to disadvantage. Further, the addition of additives not only complicates the operation but also adds to the level of impurities.
It is known that in hydrogenation reactions the atomic hydrogen produced on the catalyst acts to accelerate the reaction regardless of whether the catalyst is uniform or nonuniform.
Another conventional method for effecting a hydrogenation reaction efficiently and safely comprises supporting hydrogen on palladium or another hydrogen-occluded alloy and contacting the same with a reactant to be hydrogenated (K. Ohkawa, K. Hashimoto, A. Fujishima, Y. Noguchi and S. Nakayama, "J. Electroanal. Chem.", [345], 445 (1993)). Palladium and most of the above hydrogen-occluding alloys exhibit catalytic action in these reactions. Hydrogen in palladium or other hydrogen-occluding metals acts as an active hydrogen having strong reactivity. It is thus considered that palladium or the like acts as a hydrogen source and a hydrogenation catalyst to exhibit high performance in the hydrogenation of organic compounds. However, since the amount of hydrogen which can be occluded in these alloys is limited, the hydrogenation reaction in the presence of palladium or a hydrogen-occluding alloy no longer proceeds as the occluded hydrogen is used up, leaving part of the organic compound unreacted. Therefore, such a hydrogenation reaction must be carried out batchwise. This batchwise hydrogenation reaction brings about no problem on a laboratory basis. However, this discontinuous operation is extremely inefficient on an industrial scale.
In order to overcome these difficulties, the present inventors proposed an electrolytic method comprising conducting electrolysis in an electrolytic cell comprising an anode and a cathode made of a hydrogen-occluding material. The reactant is in contact with the side of the cathode opposite the anode and atomic hydrogen produced on the cathode is occluded by the cathode. The reactant is hydrogenated by atomic hydrogen which permeates through the cathode to the side opposite the anode (JP-A-9-184080). However, the arrangement comprising as a partition a metal plate such as palladium is disadvantageous in that palladium or the like is expensive. A metal foil, if any, is disadvantageous in that it can easily break if used over a large area.